In an IP network, a traffic exceeding the data forwarding capacity (band) of a packet forwarding apparatus is expected to be inputted in the packet forwarding apparatus of routers, switches, and the like for relaying packets. For this reason, a packet buffer for temporarily storing packets is provided in a packet forwarding apparatus, and normally a traffic exceeding the transferable capacity of the apparatus is controlled to be held in the packet buffer.
Meanwhile, if a packet including data of highly real-time audio data, motion picture data, or the like, is held for a long period of time in the packet buffer and is transferred, this packet may be discarded at the side of a receiving terminal that is a subsequent stage of the packet forwarding apparatus. This is because the reproduction of the data included in the packet that arrives late is sometimes judged to be unnecessary in a higher layer at the receiving terminal for processing the highly real-time data. This means that, by the packet being sent late due to the packet forwarding apparatus, the band of the network from the packet forwarding apparatus to the receiving terminal is used purposelessly, and it becomes a serious problem especially if the network is congested.
In order to avoid the problem concerned, a control is performed for measuring the residence period of packets in a packet forwarding apparatus, and discarding the packets with residence periods exceeding a predetermined threshold. As a conventional method for controlling and discarding packets, with a packet exchange equipment (packet forwarding apparatus) disclosed in Japanese Laid-open Patent Publication No. 05-207063, the difference between the arrival (input) time and transmit (output) time of a packet (in other words, the residence period of a packet) is described in a control field of packets, and based on the control field, an apparatus (“PAD” in the publication) of a subsequent stage of the packet exchange equipment discards packets that delayed for a predetermined period of time.